(*Objék nu minuhan sarat (i) jeung (ii) (kadang-kadang disebut planémo) sarta ngambang di angkasa otherwise than in a young star cluster are not currently defined, as their status is in dispute within the astronomical community. No such objects are known to exist.)

This summary of the IAU's position is based on two definitions released by the Union. Although their stance on planets within our solar system has been formal adopted by the IAU general assembly in 2006, the position on extrasolar planets remains only a working definition in place since 2003. As such, it is highly subject to change until it is formally adopted through due process.

Before the adoption of the 2006 resolution, there was no formally specified scientific definition of "planet", even for our own system. Without one, the solar system had been traditionally seen as having various numbers of accepted planets over the years. It remains to be seen whether the new definition will become universally accepted both within the astronomical community, where there has been opposition to the recent adoption, and in the eyes of the greater public.

In near-universal practice in the Western world, the planets in the solar system are named after Graeco-Roman gods, as, in Europe, it was the Greeks who originally named them. The name "planet" comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. These objects were believed to orbit the Earth, which was considered stationary.

However, due to the influence of the Roman Empire and, later, the Catholic Church, in most countries in the West, the planets are known by their Roman (or Latin) names, rather than the original Greek. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable. When the Romans studied Greek astronomy, they gave the planets their own gods' names. The Greeks still use their original names for the planets. In ancient times, there were seven known planets; they were, in order from the supposedly central Earth: the Moon (called Luna by the Romans, and Selene by the Greeks), Mercury, (called Hermes by the Greeks) Venus, (Aphrodite), the Sun (Called Sol by the Romans, Helios by the Greeks), Mars, (Ares) Jupiter, (Zeus) and Saturn (Kronos). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptune (Poseidon).

Some Romans, following a belief imported from Mesopotamia into Hellenistic Egypt,[3] believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth, in Ptolemaic orbit order listed inwards. As a result. a list of which god has charge of the first hour in each day came out as Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order.[4] Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, Wóden, Thunor, and Fríge, Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus.

Since Earth was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god. However, many of the Romance languages (including French, Italian, Spanish and Portuguese), which are descended from Latin, retain the old Roman name of Terra or some variation thereof; again, the Greeks retain their original name, Γή (Ge or Yi; derived from Gaia). However, the non-Romance languages use their own respective native words. The Germanic languages, including English, use a variation of an ancient Germanic word ertho, "ground," as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. The same is true for the Sun and the Moon, though they are no longer considered planets.

Some non-European cultures use their own planetary naming systems. China, and the countries of eastern Asia subject to Chinese cultural influence, such as Japan, Korea and Vietnam, use a naming system based on the five Chinese elements.[4]

Developments in astronomy meant the removal of the Sun and the Moon, and the gradual addition of the currently accepted planetary members of the Solar System. Although a "planet" was commonly considered to be a large object orbiting the Sun (or, later, other stars), there was no scientifically specified definition of "planet". Since the acceptance of the heliocentric model over the geocentric model, the solar system has been seen as having various numbers of accepted planets over the years, based on culture and scientific consensus:

1543 - six (Mercury, Venus, Earth, Mars, Jupiter, Saturn) (among those who accepted the new view)

When the objects Ceres, Pallas, Juno and Vesta were found orbiting between Mars and Jupiter in the early 1800s, they were declared and accepted as planets, and this remained the case for many years. However, as more and more objects began to be found in the same region of the solar system, they became classified as asteroids, along with their orbital kin. Ceres' relative large size and degree of roundness meant that some have recently argued for it to qualify as a planet once again, but in 2006 it was placed by the International Astronomical Union in the new category of dwarf planets.

One of these objects was Eris, whose 2003 discovery was widely (and preemtively) reported in blogs, journals and the mainstream media as "the tenth planet", although the IAU never formally accepted the body. Many lobbied for Eris to be included in the list of planets, but after prolonged debate it joined Ceres and Pluto as a dwarf planet.

In 2003, The IAU Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated a working definition:[2]

1) Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.

2) Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.

3) Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate.

This definition has been widely used by astronomers when publishing discoveries in journals since this time, although it remains a temporary, working definition until a more permanent one is formally adopted. It also did not address the controversy over the lower mass limit.

Under this definition, the solar system is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto and Eris) are classified as dwarf planets, providing they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets because it did not include (c) as a criterion. Negotiations resulted in those bodies being reclassified as dwarf planets.

This definition is based in modern theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects.

A paper by Steven Soter describes it this way:

The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Asteroids and comets, including KBOs, differ from planets in that they can collide with each other and with planets.[5]

Planetary discriminants defined in that paper very clearly separate the 8 IAU-recognized planets from all other small solar system objects.

In the aftermath of the IAU's 2006 vote, there has been some criticism of the new definition from astronomers. The dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition. The next IAU conference is not until 2009, when modifications could be made to the definition, also possibly including extrasolar planets.

Beyond the scientific community, Pluto has held a strong cultural significance for some of the public (given its planetary status since 1930), in a similar way to Ceres and its kin in the 1800s. More recently, the discovery of Eris (widely publicised as the tenth planet) was widely reported in the media as the "tenth planet". The reclassification of all three objects as dwarf planets, has attracted much media and public attention.

It is not known with certainty how planets are formed. The prevailing theory is that they are formed from those remnants of a nebula that do not condense under gravity to form a protostar. Instead, these remnants become a thin, protoplanetary disk of dust and gas revolving around the protostar and begin to condense about local concentrations of mass within the disc known as planetesimals. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets.[6] After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere. This serves to increase the capture rate of the planetesimals by a factor of ten.[7]

When the protostar has grown such that it ignites to form a star, its solar wind blows away most of the disc's remaining material. Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.[8][9] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small solar system bodies.

The energetic impacts of the smaller planetesimals will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core. Smaller terrestrial planets lose most of their atmospheres due to this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.[10] (Note that smaller planets will lose any atmosphere they gain through various escape mechanisms.)

With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity is now believed to determine the likelihood that a star will have planets.[11] Hence it is thought less likely that a metal-poor, population II star will possess a substantial planetary system than a metal-rich population I star.

All planets revolve around the Sun in the same direction; counter-clockwise as seen from over the Sun's north pole. The period of one revoultion of a planet's orbit is known as its year. A planet's year depends on its distance from the Sun; the farther a planet is from the Sun, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the Sun's gravity.

All planets also rotate around invisible axes at their centres. The period of one rotation of a planet is known as its day. Most planets, including the Earth, rotate as they revolve, in a counter-clockwise direction. Venus, alone among the planets, rotates clockwise. If it were possible to see the Sun from the surface of Venus, it would rise in the west and set in the east. There is great variation in the lengths of the days between the planets. Mars's day is only 37 minutes longer that Earth's; Venus's is far longer, at 243 days. The gas giants all have rotational periods lasting only a few hours.

Planets also have varying degrees of axial tilt; they lie at an angle to the ecliptic plane. This causes the amount of sunlight received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from the Sun, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses seasons; changes to the climate over the course of its year. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.

Before the August 2006 debate, many objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets. Dwarf planets are not planets, according to the IAU. Currently three dwarf planets in the Solar System are recognized by IAU, those are Ceres, Pluto and Eris. Dwarf planets share many of the same characteristics as planets, although notable differences remain. Their attributes are:

The large bodies within Earth's solar system can be divided into categories according to composition.

Terrestrials (or rock planets): Planets (and possibly dwarf planets) that are similar to Earth — with bodies largely composed of rock: Mercury, Venus, Earth and Mars. If including dwarf planets Ceres would also be counted, with at most three other asteroids that might be added.

Gas giants (or Jovian planets): Planets with a composition largely made up of gaseous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Uranian planets, or ice giants, are a sub-class of gas giants, distinguished from true Jovians by their depletion in hydrogen and helium and a significant composition of rock and ice.

Ice dwarfs (or Plutonian objects): These are objects that are composed mainly of ice, and do not have planetary mass. The dwarf planets Pluto and Eris are ice dwarfs, and several dwarf planetary candidates also qualify.

Of the 207 extrasolar planets (those outside our solar system) discovered to date (September 2006) most have masses which are about the same as or larger than Jupiter's.

Exceptions include a number of planets discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12,[12] the planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized,[13] and a planet orbiting Gliese 876 that is estimated to be about 6 to 8 times as massive as the Earth and is probably rocky in composition.

It is far from clear if the newly discovered large planets would resemble the gas giants in our solar system or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in our solar system, which makes it questionable whether they are the same type of planet at all. There is also a class of hot Jupiters that orbit so close to their star that their atmospheres are slowly blown away in a comet-like tail: the Chthonian planets.

In 2005, astronomers[14] detected a planet in a triple star system, a finding that challenges current theories of planetary formation. The planet, a gas giant slightly larger than Jupiter, orbits the main star of the HD 188753 system, in the constellation Cygnus, and is hence known as HD 188753 Ab. The stellar trio (yellow, orange, and red) is about 149 light-years from Earth. The planet, which is at least 14% larger than Jupiter, orbits the main star (HD 188753 A) once every 80 hours or so (3.3 days), at a distance of about 8 Gm, a twentieth of the distance between Earth and the Sun. The other two stars whirl tightly around each other in 156 days, and circle the main star every 25.7 years at a distance from the main star that would put them between Saturn and Uranus in our own Solar System. The latter stars invalidate the leading hot Jupiter formation theory, which holds that these planets form at "normal" distances and then migrate inward through some debatable mechanism. This could not have occurred here; the outer star pair would have disrupted outer planet formation.

The term is a controversial one. A prominent school of astronomers, including the IAU, argue that only objects that orbit stars qualify as planets and thus an "interstellar planet" is an oxymoron. The discoverers of the bodies mentioned above decided to avoid the debate over what constitutes a planet by referring to the objects as planemos. See also definition of planet.